The present invention relates to electrochemical fuel cells and a method of operating electrochemical fuel cells. More specifically, the present invention relates to polymer electrolyte membrane (PEM) fuel cells employing fuel and oxidant streams. More particularly, the present invention relates to PEM fuel cells utilizing hydrogen as the fuel and oxygen containing air as the oxidant.
Electrochemical fuel cells convert fuel and an oxidant to electricity and a reaction product. A typical fuel cell consists of a cathode, an anode, and an electrolyte. The electrolyte is sandwiched between the cathode and anode. Fuel, in the form of hydrogen, is supplied to the anode where a catalyst (usually platinum) catalyzes the following reaction: EQU Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.-
Hydrogen separates into hydrogen ions and electrons. The cations (protons) migrate through the electrolyte membrane to the cathode. The electrons migrate via an external circuit in the form of electricity.
An oxidant, in the form of oxygen or oxygen containing air, is supplied to the cathode where it reacts with the hydrogen ions that have crossed the membrane and the electrons from the external circuit to form liquid water as the reaction product. The reaction is also usually catalyzed by platinum and occurs as follows: EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
Thus the fuel cell generates electricity and water through the electrochemical reaction.
Typically, the electrochemical reaction also supports a phenomenon called water pumping. As each cation (proton) migrates through the membrane, it transports or drags along several water molecules, too. Thus, there is a net transport of water to the cathode. Water pumping adds water to the product water formed at the cathode through the electrochemical reaction.
Solid polymer fuel cells generally comprise a membrane-electrode assembly (MEA). The MEA consists of a solid polymer electrolyte or ion exchange membrane situated between and in contact with two electrodes made of porous, electrically conducting sheet material. The electrodes are typically made from carbon fiber paper or cloth. At the interface of the electrode and membrane is a layer of catalyst to facilitate the electrochemical reaction. The MEA is placed between two electrically conductive plates, commonly formed from graphite. These plates have one or more reactant flow passages impressed on the surface. The reactant flow passages direct the flow of a reactant to the electrode.
The graphite flow plates, one for each electrode, provide support for the MEA and act as current collectors. Additional cells can be connected together in series to increase the voltage and power output. Such an arrangement is referred to as a fuel cell stack. In such an arrangement, the current collector plate will act as a flow plate for the anode on one side and as the flow plate for the cathode on the other. The stack will typically include inlets, outlets, and manifolds for directing the flow of the reactants as well as coolant, such as water, to the individual reactant flow plates.
To date, fuel cells have been utilized as prototypes for research or in very specialized, custom applications. In order to successfully commercialize fuel cells for vehicular propulsion, the fuel cell stack should be sized to provide sufficient power, at a useful voltage, for normal continuing operation. A significant barrier to commercialization is the power density of the fuel cell; in other words, the power produced for the volume or unit mass of the fuel cell stack. Power density needs to be higher to be practical for transportation. As a corollary, increased power density will decrease the cost to manufacture a given power rating. Thus, there is considerable interest in industry to increase fuel cell power density.
It is known that the performance of the fuel cell or fuel cell stack typically increases with increasing pressure of the reactants. As reactant pressure increases, the power density of the fuel cell increases. See for example Srinivasan, et al., "Advances in Solid Polymer Electrolyte Fuel Cell Technology with Low Platinum Loading Electrodes" (Journal of Power Sources, 22 (1988) 359-375) in which this proposition is discussed. Fuel cell systems such as those disclosed in U.S. Pat. Nos. 5,260,143, 5,252,410, 5,366,821, and 5,360,679 and 5,346,778 utilize reactant pressures ranging from 10 p.s.i.g. to approximately 65 p.s.i.g. or higher. A common emphasis in these systems is to maximize the power output of the fuel cell or fuel cell stack.
Further, papers such as "The Renaissance of the Solid Polymer Fuel Cell" (Journal of Power Sources, 29 (1990) 239-250) by K. Prater and "Solid Polymer Fuel Cell Developments at Ballard" (Journal of Power Sources, 37 (1992) 181-188) also by K. Prater, disclose reactant pressures ranging from 30 psig to 50 psig and higher for improved cell and stack performance.
Many of the above systems use (substantially) pure oxygen as the oxidant. However, when working with oxygen-containing air as the oxidant reactant, the reactant must typically be supplied in larger volumes to compensate for the reduced concentration of oxygen. The increased flow is required to maintain sufficient stoichiometric conditions to support the electrochemical reaction.
A consideration in the operation of a fuel cell stack is the power diverted to supporting apparatus. Supporting apparatus includes pumps, compressors or blowers, control systems, and other equipment. The power to operate these sub-systems represents a drain on the net available power from the fuel cell. This is commonly referred to as "parasitic losses" or "parasitic load." The parasitic load required to run the subsystems varies, directly, with the gross power output of the fuel cell stack. As the net power demand increases, the gross output of the fuel cell stack increases and the parasitic load also increases.
The parasitic load for oxidant compression, or the means for moving an oxidant through the fuel cell, for fuel cells operating with air as the oxidant can represent a significant portion of the total parasitic load. U.S. Pat. Nos. 5,366,821, 5,366,818, and 5,292,600 recognize the impact of such a drain and in these references the inventors considered minimizing the parasitic losses of the fuel cell system due to the air moving means.
Further references can be found in Swan, et al., "Proton Exchange Membrane Fuel Cell Characterization for Electric Vehicle Applications" (1994 SAE International Congress, February 28-March 3, Detroit, Mich.); Swan, et al., "The Proton Exchange Membrane Fuel Cell--A Strong Candidate as a Power Source for Electric Vehicles" (Technical Proceedings--Project Hydrogen '91, American Academy of Science, Independence, Mo.); and Amphlett, et al., "Operating Characteristics of a Solid Polymer Fuel Cell" (Technical Proceedings--Project Hydrogen '91, American Academy of Science, Independence, Mo.). The authors demonstrate that compressor power requirements can constitute a major parasitic load that is not offset by improved net efficiency.
FIG. 1 shows the minimum voltage loss and the fractional net power loss due to adiabatic compression under ideal conditions. Fuel cells operate under real world conditions, not "ideal" conditions. Ergo, the actual losses will be higher.
Therefore, with respect to fuel cell design, a paradoxical problem exists; namely, fuel cell performance increases with increasing reactant pressure while parasitic losses similarly increase which serves to negate or even reduce net power gains.
Another consideration in fuel cell stack performance concerns water management. The accumulation of water at the cathode, due to water pumping and from product water, creates problems for the operation of the fuel cell. The presence of water in the vicinity of the catalyst layer reduces the accessibility of the catalyst to the reactant, a phenomenon commonly referred to as "flooding." Also, the presence of water, often in the form of droplets, can substantially block the flow of oxidant reactant through the oxidant passages. "Dead spots" can form in the areas where channel passages are blocked. These issues can result in a reduction of power of the fuel cell.
U.S. Pat. Nos. 5,292,600, 5,108,849, 4,855,193, and 4,729,932, recognize the need to maintain adequate oxidant flow in cathode flow passages to adequately remove water from the cell. The flow of the oxidant stream clears the water in the passages. An adequate flow of oxidant is necessary to keep all flow passages clear and to eliminate any potential dead spots that could reduce the power of the system. Maintaining adequate oxidant flow typically increases the required parasitic power. Thus, an issue with water management is the increase in parasitic losses to provide sufficient oxidant flow through the channels.
Another consideration in fuel cell stack performance that results from the oxidant air flow is that of pressure drop. Air or oxidant flowing through the reactant flow passages results in a pressure drop from the reactant inlet to the reactant outlet. The pressure drop is induced by the need to maintain adequate reactant flow for the previously mentioned water management. U.S. Pat. Nos. 5,108,849 and 4,988,583 recognize that pressure drop from reactant inlet to reactant outlet across a cell becomes significant, particularly in larger size fuel cells utilizing air as a reactant or in fuel cell stacks. Increased pressure drop across the cell requires increased inlet pressure for the oxidant and, thus, increased parasitic power.
It is an object of the present invention to provide a fuel cell that maximizes the net power from the fuel cell system. It is a further object of the present invention to reduce the overall pressure of the fuel cell system to minimize parasitic losses while still maintaining adequate airflow through the flow channels to remove water. It is also an object of the present invention to minimize the pressure drop across a fuel cell and through the fuel cell stack to reduce the parasitic losses.